It has long been desirable to remove pollutants from water in a safe, efficient and cost-effective manner.
Agricultural, industrial, and street run-off, among other polluted water flows, require treatment before being released into the environment. The high concentration or synthetic chemistry of these wastes can overwhelm self-purifying mechanisms in the receiving environment. When this occurs, the result is contaminated ground water and/or surface water.
In high-density population areas, it is typical to have a public waste water treatment system. In less densely populated areas, where public sewage treatment is not available, many homes and businesses use a septic system, implemented on-site, for the treatment and disposal of waste water or sewage. A typical on-site waste fluid treatment system or septic system includes a mound or drain field portion and a septic tank. Waste fluids may include such things as waste from washers and dryers, showers and bathtubs, toilets, disposals, disposal waste, sink waste water and commercial operations. In a typical on-site septic system, the waste water generally drains into a septic tank before being routed to the mound or drain field. A septic tank typically functions to separate the solid and liquid fractions of the waste water, and then typically introduces the effluent back into the ecosystem with significant and undesirable nutrients and other pollution.
The average life of a conventional on-site waste water treatment system is typically only seven to 10 years. A failing system can lead to public health concerns and non-point source pollution. Another concern of the conventional on-site water treatment facility beyond its finite life, is the inability to quickly assess the proper functioning of the system. Untreated waste water may be leaking into the ecosystem with little or no surface indication.
A primary concern with any on-site septic system or waste water treatment system is to ensure that nutrients and other pollutants are removed from the waste water before the waste water is introduced back into nature. If the water is not sufficiently pollutant-free, the effluent may contaminate surface or subsurface water creating water quality problems.
Natural wetlands have been used as waste water discharge sites for a long period of time, and the ability of wetland plants to remove pollutants from waste water is fairly well known. Constructed wetlands have made limited use of natural wetland potential.
Existing constructed wetlands, including surface water and subsurface flow systems, utilize wetland plants and atmospheric diffusion to transfer oxygen into the water column. These naturally aerated (aerobic) zones support populations of bacteria which require oxygen to support their metabolisms. Other areas within the constructed wetland are anaerobic, and support populations of bacteria which do not require oxygen. It is known that aerobic metabolic pathways are much more efficient than anaerobic pathways. Consequently, aerobic bacteria are capable of consuming, and thus removing, more of a pollutant than anaerobic bacteria for a given treatment cell size. In existing constructed wetlands, aerobic zones are typically only found at the top of the water column, in those regions where there is sufficient atmospheric diffusion, and in the immediate vicinity of wetland plant root hairs, where oxygen transported by wetland plants diffuses out through the root membrane. Therefore, in current constructed wetland systems, aerobic zones occupy only a small fraction of the constructed wetland. This lack of aerobic zones places a constraint on the overall treatment capacity of the wetland, particularly in subsurface flow constructed wetlands.
Accordingly, it would be desirable to more fully utilize the pollution and nutrient reducing characteristics of wetland plants in a constructed system to treat polluted water.